1) it really all depends on your MIL liao .... cause buy the si dian jin is from her pocket.... rich one will give 4 diamonds instead ... I dun think anyone will buy silver la ...
Some bride even ask their MIL wat is their budget and they the bride top up the difference as they wanted something more expensive. So do check with your MIL her budget ....(basically ask your htb la i think he will have an idea)

2) This si dian jin is to be given to you on the guo da li day

3) This si dian jin is normally wear on your AD during your tea ceremoney ... some MIL they very easy going as in if you dun want they dun mind you dun wear. .... some very traditional .... so do check it out ...(again ask your htb to check )

GDL depends if you need to choose a good date. normally if you practice GDL, means you will need to find an auspicious date. Else, it can be anytime from 1 month to 1 week before.
mostly ppl do it 1 month in advance so you have distribute the cakes and pork to relatives when giving the invitation cards. cos the pork is given during the GDL..